Mutations in a broad sense include all those heritable changes, which alter the phenotype of an individual. Hugo de Vries used the term 'mutation' to describe phenotypic changes which were heritable. He is, therefore, credited to have differentiated between heritable and environmental variations. However, the term mutation is now used in a rather strict sense to cover only those changes which alter the chemical structure of the gene at the molecular level. These are commonly called gene mutations or point mutations. In practice, sometimes it is rather difficult to distinguish between gene mutations and structural changes in chromosomes, because certain structural changes may have same phenotypic effects as gene mutations. Small deficiencies cannot be discovered by cytological observations. Although in Drosophila small deficiencies can also be detected in giant salivary gland chromosomes, in other organisms the only test for a deficiency is that it will not revert back to wild type character. However, gene mutations would be able to give reverse mutations.